This invention relates to a method for making a roll-threaded, pointed screw and more particularly, a method using a blank having a hollow end as the workpiece from which such a screw is formed.
A variety of methods are known for producing pointed threaded fasteners, such as wood screws or sheet metal screws. Some of the most widely used methods include forming the threads by progressive movement of a blank through thread-forming dies as described in Gordon, U.S. Pat. No. 3,196,654; Johnson, U.S. Pat. No. 2,162,891; and DeVellier, U.S. Pat. No. 2,314,391.
One method of making a roll-threaded pointed fastener utilizes a blank having a conical point and then rolling the threads thereon, but this method has disadvantages which may include removal of metal to form the conical end and there may also be difficulty in providing sufficient metal in the conical end to form a thread continuously to the tip. Johnson, U.S. Pat. No. 2,162,891, overcomes the problem of metal removal by die forming a generally tapered end during a cold-heading step. Johnson successively forms decreasing diameter cylinders on the end of the blank and thus provides a stepped, tapered screw blank without removing metal.
DeVellier, U.S. Pat. No. 2,314,391, describes a method that is particularly adapted for roll-forming a double threaded pointed fastener. DeVellier utilizes a blank having a generally tapering point with excess metal provided in the generally tapering point portion so that there is sufficient stock to continuously form threads to the point of the fastener.
Gordon, U.S. Pat. No. 3,196,654, eliminates the use of a pointed or tapered blank. A uniform diameter stock is cold-headed in a conventional manner and the headed blank progresses through a thread-forming die of the patentee's invention. The die is so constructed that as the blank progresses through the die, an end portion of the blank is coincidentally threaded and tapered. The excess metal from the blank is extruded outwardly from the tapered portion as the tapering continues until the tip of the screw is formed at which point a configuration within the die causes the excess extruded metal to break off. Although this method eliminates a separate tapering step, it is difficult to use in producing a consistently good point on fasteners made of certain metals. Since the tapering of the end is done mechanically in a die, the metal is significantly cold worked in the tapered portion and this can cause excessive embrittlement of some metals in the zone of the prospective point of the fastener and thus produce a jagged or irregular point when the excess metal is broken away as just described.
Many of the tempered aluminum alloys commonly used for making threaded pointed fasteners, for example, are susceptible to embrittlement when subject to the degree of cold work required to produce such a fastener by threading and pointing a uniform diameter screw blank. Therefore, to insure producing a consistently good point on threaded pointed fasteners made from many of the commonly used aluminum alloys, it has been necessary to either machine the threads and point which is slower than thread rolling and generates excess scrap, or to pretaper the blank before thread rolling. In either case, the cost of the finished fastener is greater than a fastener produced from processing a blank having a uniform diameter shank through thread rolling and pointing equipment.
It is desirable, therefore, to provide an improved method for producing a pointed threaded fastener having a consistently good point by thread rolling and tapering a uniform diameter blank through a die.